1. Field of the Invention
The present invention relates to an amplification circuit and in particular relates to an amplification circuit that selectively inputs an input signal to an amplifier and selectively bypasses an amplifier.
2. Description of the Related Art
There are amplification circuits, which are for an input signal received from an antenna or the like, in which processing is performed to cause an input signal to be input to and amplified by a low-noise amplifier in the case in which the input signal is a low-power signal and to cause the input signal to bypass the low-noise amplifier in the case in which the input signal is a high-power signal.
To date, a variety of amplification circuits equipped with a bypass circuit have been proposed that selectively perform low-noise amplification and bypassing in this manner (see, for example, Japanese Unexamined Patent Application Publication No. 2014-27501).
Japanese Unexamined Patent Application Publication No. 2014-27501 discloses a variable gain amplification circuit that includes a low-noise amplifier (hereafter, “amplifier”) that is connected between an input matching network and an output matching network and a bypass circuit that is connected in parallel with the amplifier in order achieve input/output impedance matching regardless of the gain control state while maintaining low-noise amplification performance. In this variable gain amplification circuit, the output matching network matches the impedance on the output side of the amplifier when a high gain mode is selected and a matching correction circuit is connected in parallel with the output matching network and matches the impedance on the output side of the bypass circuit when a low gain mode is selected.
The variable gain amplification circuit of Japanese Unexamined Patent Application Publication No. 2014-27501 is also able to cause an input signal to be input to and amplified by the amplifier when the input signal is a low-power signal and to cause the input signal to bypass the amplifier when the input signal is a high-power signal.
However, there is a problem with the variable gain amplification circuit of Japanese Unexamined Patent Application Publication No. 2014-27501 in that the input signal is affected by the frequency characteristics of the input matching network when bypassing is performed and therefore the pass band characteristics have a high frequency dependence when input signals over a wide frequency band (that is, input signals of a plurality of frequency bands) are handled. That is, although the input matching network is provided in order to match the impedance on the input side of the variable gain amplification circuit, the input signal is input to the bypass circuit without being input to the amplifier after passing through the input matching network when bypassing is performed. Therefore, for an input signal of a frequency that is outside the pass band of the input matching network, pass loss occurs at that frequency when bypassing is performed. As a result, the pass loss that occurs differs depending on the frequency of the input signal and makes it difficult to use the circuit as a reception circuit that handles input signals of a plurality of frequency bands.
Furthermore, in the variable gain amplification circuit of Japanese Unexamined Patent Application Publication No. 2014-27501, when bypassing is performed, although the amplifier is turned off, the bypass circuit and the input terminal of the amplifier are connected in parallel with each other and therefore the input signal is also affected by frequency characteristics due to the input impedance of the amplifier. Therefore, there is also a problem in that the difference in pass loss increases depending on the frequency of the input signal.
Here, changing the input matching network in order to improve the frequency dependence of the bandpass characteristics when bypassing is performed may also be considered (for example, changing the input matching network itself, changing the number of elements making up the input circuit, changing the elements themselves, changing the constant values of the elements and so forth). However, if such a change was made, the input matching network would no longer be the optimal matching network for the amplifier. Consequently, the characteristics when an input signal is amplified by the amplifier (frequency dependence of bandpass characteristics) would no longer be secured.